Brinell Hardness Test – Useful Tool for Metallurgist

Introduction

Hardness is a very important property of the material as it defines materials’ strength to resist plastic deformation. Based on the hardness of materials, they are being employed in applications where deformation resistance is required like in coatings. Tests commonly employed for Hardness testing are Brinell hardness test Rockwell hardness test, Vicker hardness test, and Knoop hardness test.

Brinell hardness testing is most commonly employed destructive testing method. The principle of test is to press the indenter on surface of sample with such a force that It leaves the indent. Brinell hardness number is denoted with HBN. A softer material, larger the depth of indentation and smaller the HBN.

Brinell Hardness Test Principle

Brinell Hardness test works according to the ASTM E10 standard. Mostly this test is employed for relative materials having coarse microstructures or rough surfaces.

The impression is made on a sample by indenting the surface of a sample with a 10mm diameter steel ball for 10 to 30 seconds and, later on, measuring the diameter of an impression after the test.

Construction of Brinell hardness test

The Brinell Hardness Tester is composed of

• Loading system
  • Main Screw
  • Dial Gauge

• Brinell hardness test used machine consisting of Loading system, Main Screw and Dial gauge.
• Loading system mainly consists of weights, levers, hydraulic dashpot and plunger arrangements.
• Loading system is enclosed in cast iron body.
• Sample holder is placed on main wheel for rotating and adjusting sample position.
• Tungsten carbide or hardened steel balls are used for indenting the specimen.

The loading system consists of:

• Weights
• Levers
• Hydraulic dashpot
• Plunger arrangement

These are enclosed in the cast iron body of the machine. The main screw might get damaged from extraneous elements so it is protected by a rubber bellow. The main wheel carries the test table on its top to hold the specimen. Steel or Tungsten carbide balls are used as indenters.

For US Customers

Indenters for Brinell Hardness Test

Depending upon materials hardness, either steel balls or tungsten carbide balls are used. Classification of materials is as follows:

Hard Materials: Hardened steel balls having diameter of 10mm are used as indent. Load applied in this case is 3000kg.

Soft Materials: Hardened steel balls having diameter of 10mm are used as indent. Load applied in this case is 500kg to avoid deep indent.

Extremely Hard Materials: Tungsten carbide balls having diameter of 10mm are used as indent to avoid distortion. Load applied in this case is 3000kg.

Time for Indentation

Similar to indent types, time for indentation is also important. For uniform distribution of load over sample and complete distortion of sample, essential time should be between 10 to 30 sec. Classification based on time is as follows:

• For hard and extremely hard materials like cast iron and steel, the time of indentation should be no more than 10 seconds.
• For softer materials like Cu and Aluminum alloys, time for indentation is essentially 30 seconds.
• For Extremely soft materials like Aluminum, time is little increased to 35 seconds for indentation.

Measurement of Indent Diameter

Travelers microscope is used in laboratory for measuring diameter of indent as it is portable and easy to use.

Traveler’s microscope is used by placing specimen in holder of microscope. Diameter is measured in two directions with length scale shown on sides. Two sides diameter is than averaged for accurate diameter of indent.  

Brinell Hardness Test Procedure

1. Suitable steel and copper specimen were selected.
2. These samples were ground in order to remove grease or any foreign material.
3. The sample was then placed on the anvil of the testing machine.
4. Steel ball indenter was inserted properly.
5. The workpiece was then raised by a turning handwheel until it contacted the steel ball indenter.
6. The pressure valve was then closed.
7. After it, the load was applied hydraulically.
8. This load was maintained for a specific period of time.
9. Then the pressure valve was opened and the load was released.
10. The sample was removed.
11. The diameter of the impression was then measured by using a traveler microscope. Two such readings were taken at right angle to each other and the mean diameter was recorded.
12. Brinell Hardness Number (BHN) was calculated finally with the help of this formula:

where;

P = Applied Load

D = Diameter of Indenter d = Diameter of indent

Examples of various samples

• Diameter of Steel Ball Indenter = D = 10mm

For Travelling Microscope

• No. of divisions between two large divisions = 20
• Each small division = 0.05mm

For Steel

Load = P = 3000kg Time = 10 seconds

Diameter of indent = d = 3.75mm

HB =262 (262 HB 10/3000/10)

For Copper

Load = P = 1000kg Time = 10 seconds

Diameter of indent = d = 3.2mm

HB = 121.13               (121.13HB 10/1000/30)

Indentation Defects

There are certain defects accosiated with surface of sample which causes variation in hardness i.e. Ridging and sinking in.

Piling up/Ridging

      Materials, when cold worked, get extremely hard. In those cases, when indent made an impression, indentations edges piled up. This results in a difference in actual diameter and piled up impression. The defect of such type is called piling up/ridging.

Sinking In

In the case of annealed structures, materials get extremely soft. In those cases, if hardened steel balls are used for a long time like 30 seconds, then edges of impressions get sunk in which increases the diameter of an impression than actual indent diameter. Such a defect is called a sinking defect.

Brinell Hardness and Mechanical Properties

Hardness is related to the tensile strength of the materials.

• Tensile strength (UTS) MPa= HB x 3.54 (annealed PCS)
• Tensile strength (UTS) MPa = HB x 3.256 (quenched hardened and tempered steel)
• Tensile strength (UTS) MPa = HB x 5.6 (Cu-alloys)
• Tensile strength (UTS) MPa = HB x 3.54 (wrought iron)

Hardness of annealed plain carbon steels is low because during annealing process the heating rate is low, holding time is high which results into the grain coarsening. Where in case of quenched and tempered steel hardness is high due to the formation of martensite during fast cooling. In martensite, amount of carbon is high which is responsible for hardness of material.

Advantages of Brinell Hardness Test

Following are the advantages of Brinell hardness test:

• It gives us an average value of different phases and defects in a material.
• It can be directly related with UTS of material.
• The results of Brinell hardness test are independent of force.
• The test can be performed on inhomogeneous materials.
• This test is less influenced by surface scratches and roughness than other hardness test.
• All the metals can be tested by Brinell hardness test.
• The test procedure is simple.
• Indenter is simple and cheap.
• Thick and hard samples can be tested. The harder the material greater will be the hardness number.

Limitations of Brinell Hardness Test

• It cannot be used for thin and small samples. Thickness should be 10 times of indentation depth.
• Test limits in critically stressed parts indentation could be the site for failure.
• Applications range to a maximum Brinell hardness of 650 HBW.
• Indent should be far away from edges.
• It cannot be used for very hard and very soft materials.
• It leaves a noticeable indentation which is objectionable for finished product.
• The edge of the indentation is not always clearly defined. And may be difficult to observe accurately.